In scientific research, as well as in other applications, it is often desired to illuminate an object of interest, such as a biological specimen, with filtered light, or to filter the light collected from the object. As a simple example, a specimen may be dyed with a florescent substance or marker and illuminated by light of a particular wavelength to effectuate the desired florescence. Likewise, it is often desirable to observe light emissions from an object under investigation through a filter, for example, when the light emitted from the object comprises a specific characteristic wavelength. Thus, the experimenter may wish to irradiate the object with filtered light to excite a florescent dye and/or observe the light collected from the object through a filter to ascertain its emission characteristics.
The use of interference gratings that allow only transmission of a very narrow band of light wavelengths is a well known method of filtering light. Such filters may be referred to as bandpass filters. Typically, white light is filtered to produce the desired wavelength for illuminating the object under investigation, or collected light is filtered to eliminate everything other than light at an emission wavelength of interest. As used herein, the term “light” is intended to have broad meaning and to encompass not only visible light but also infrared and ultraviolet light subject to filtering. Likewise, as used herein, the term “white light” is intended to have broad meaning.
In many research applications, the object under investigation is very small, such that a microscope is necessary to conduct the desired experiment or procedure. In such cases, and in others, space is at a premium and it is desirable to make the various hardware systems used to conduct the investigation as compact as possible. Such hardware systems may include, for example, optical systems for illuminating a specimen, mechanical systems such as micromanipulators and the like to position the specimen, microscopy systems and probe systems for observing and making measurements of the specimen, recording systems for acquiring data and images, and control systems for operating and coordinating the hardware.
A modern optical microscope typically has an objective lens which collects the light from the object under study and a tube lens which focuses the image for viewing and/or photographic recording. It will be understood by those skilled in the art that the term “lens” as used herein comprises compound lens systems. In many modern microscopes, the objective lens collimates the light, and the space between the objective and tube lenses is sometimes referred to as the “infinity space” because the light is collimated in this region, i.e., it has a infinite focal length. While there is no theoretical limit to the length of the infinity space, as a practical matter it is impossible to attain perfect collimation of light, and so the length of the useable infinity space in commercially available microscopes is usually limited to less than about 8 centimeters. The infinity space is a particularly convenient place to insert filters, beam splitters, polarizers, etc., however any such items need to be quite compact to fit in the limited area available.
It is known that the range of bandpass frequencies of an optical interference filter shifts as a function of the angle of incidence of the light directed onto the filter. Recently, interference filters have been developed taking advantage of this property over a broad range of angles without substantial loss of the desired bandpass properties. Hence, these newly developed filters are “tunable” over a substantial range of wavelengths by changing the angle of incident light. Such filters remain useful at angles of up to 60 degrees relative to the light path. It will be appreciated that it is best if the light passing through such a filter is collimated, as is the case in the infinity space of a microscope, such that all of the rays have substantially the same angle of incidence.
Filter wheels, useful in many applications, are well known. Basically, a filter wheel comprises a plurality of optical filters mounted on a disc-shaped “wheel” that is rotatable about a central axis. By rotating the wheel any of the filters can be positioned by the user in the light path, thereby allowing the user to select (from among the filters) the wavelength of light used to illuminate the specimen. Such filters wheels are available, for example, from Sutter Instrument Company of Novato, Calif., (www.sutter.com) assignee of the present invention.
Recently, the assignee of the present invention has developed a filter wheel incorporating filters, wherein at least one, and preferable all, of the filters on the wheel are tunable. A filter wheel using tunable bandpass filters is disclosed in the assignee's U.S. application patent Ser. No. 13/162,904, the disclosure of which is incorporated by reference. While the filter wheel described in this prior application has proven successful for many applications, the inventors hereof have determined that in some instances an even more compact arrangement would be desirable, particularly for use with microscopes. In the embodiment that has been developed (as described in the above-referenced patent application), the filter wheel is rotatable about two axes, namely, a first axis that is perpendicular to the center of the wheel (as in past filter wheels), and a second axis that is in the same plane as the wheel disk and runs through the center of the disk. This second axis allows adjustment of the bandpass frequencies, in the manner described, by allowing the filter to be rotated relative to the incident light.
Quite often the same microscope may be used in connection with experiments involving more than one illumination and/or emission wavelength, either during a single experiment or in different experiments. Thus, it is desirable to be able to vary the bandpass characteristics of a filter apparatus with minimal effort and delay. Moreover, in many research applications it is beneficial to filter light at some times, while at other times to leave it unfiltered. For example, the light collected from an object undergoing microscopic examination may be left unfiltered such that is illuminated by white light when manipulating, preparing or handling the object and, thereafter, filtered to illuminate it with a selected frequency or when looking for characteristic emissions. As noted above, however, the space used for the investigation may be very limited, making difficult to place and remove filters. In order to enable an experiment to proceed efficiently, it is often desirable to quickly adjust the filter characteristics and to quickly switch between filtering and not filtering the collected light.